1. Field of the Invention
The present invention relates to an improved magnetic recording medium comprising a nonmagnetic support and a magnetic recording layer.
2. Description of Prior Art
A magnetic recording medium (hereinafter also referred to as a magnetic tape) such as an audio tape, a video tape or a recording medium used in a computer system basically comprises a nonmagnetic support and a magnetic recording layer provided on the support. The magnetic recording layer comprises a ferromagnetic powder such as a needle crystalline powder of .gamma.-Fe.sub.2 O.sub.3, Co-containing ferromagnetic iron oxide or CrO.sub.2 dispersed in a binder. Recently, a demand for a higher density recording system has increased, and hence a magnetic recording medium using a ferromagnetic alloy powder mainly containing metals such as iron, nickel and cobalt has been widely employed in place of the conventional oxide-type ferromagnetic powder. The ferromagnetic alloy powder is high in both of a coercive force (Hc) and a residual flux density (Br), so that a magnetic recording medium using such powder can afford recording of higher density. For these reasons, the ferromagnetic alloy powder can be suitably employed for magnetic recording media of higher density recording system.
Especially in a video tape, requirement for extremely high density recording has increased in accordance with utilization of measures of using a short recording wavelength or narrowing track width, and hence the ferromagnetic alloy powder has been widely used in place of the conventional oxide-type ferromagnetic powder.
In the case of using the ferromagnetic alloy powder, it is known that much higher density recording can be attained by smoothening the surface of a magnetic recording layer containing the ferromagnetic alloy powder, and thereby the resulting magnetic recording medium can be enhanced in the electromagnetic conversion characteristics. However, when the surface of the magnetic recording layer is too smooth, a friction coefficient given by the contact between the magnetic recording layer and a member of a recording device increases in the course of running of the recording medium such as a video tape. As a result, the magnetic recording layer of the recording medium is apt to be easily damaged or easily separated from the support even when the recording medium is used for a short period of time. Particularly, in the case of a video tape, since the video tape runs at a high speed in contact with a video head, the ferromagnetic powder tends to easily drop off from the recording layer in the course of running to cause occurrence of clogging on the head. Accordingly, the magnetic recording layer of the video tape is desired to be improved in running durability.
For improving the running durability of the magnetic recording layer, it has been proposed to incorporate an abrasive (i.e., hard particles) such as corundum, silicon carbide or chromium oxide into the recording layer. In this case, however, the effect of incorporation of an abrasive is shown only where the abrasive is incorporated in a large amount. However, a magnetic recording layer containing a large amount of an abrasive likely causes extreme abrasion of a magnetic head employed in contact with the recording layer. Further, the incorporation of a large amount of an abrasive into the recording layer is unfavorable from the viewpoint of enhancement of the electromagnetic conversion characteristics of the resulting recording medium which are obtained by measures of smoothening the surface of the recording layer.
It has been also proposed that a lubricant such as a fatty acid or an ester of a fatty acid and an aliphatic alcohol is incorporated into the recording layer to lower the above-mentioned friction coefficient.
Now with wide spread of a portable video tape recorder, a video tape is expected to be employed under severe conditions such as a condition of a low temperature and a condition of a high temperature and a high humidity. Therefore, the video tape is required to hardly vary in the running durability and show stable durability in the course of running even under such severe conditions. However, the above-mentioned conventional lubricants hardly improve the running durability of the recording medium to a satisfactory level.
As described before, a ferromagnetic alloy powder has been widely employed as a replacement for the conventional ferromagnetic powder with respect to a video tape or a floppy disc, as the size of such recording medium has been made smaller for ultization of a short recording wavelength or a narrow track width. The ferromagnetic alloy powder can relatively improve the electromagnetic conversion characteristics of the resulting medium but hardly improves the running durability thereof to a satisfactory level, so that the magnetic recording medium using such ferromagnetic alloy powder is desired to be improved in the running endurance.
A magnetic recording layer of a recording medium can be formed on a nonmagnetic support by a conventional coating method, that is, a method of coating a magnetic paint comprising a ferromagnetic powder dispersed in a binder over the support. Otherwise, a magnetic recording layer can be also formed on the support by a metal deposition method using no binder such as sputtering, ion plating, electroplating or electroless plating, and recently, such recording layer prepared by the metal deposition method (i.e., a metal thin film type-recording layer) has been studied in detail for practical use. However, the metal thin film type-recording layer has drawbacks concerning various properties required for magnetic recording media such as resistance to weather, smooth running property, and resistance to abrasion (running durability). For example, the metal thin film type-recording layer having an extremely smooth surface is more difficult to be provided with high running durability, as compared with the conventional recording layer of coated type. For this reason, it has been proposed to further provide a lubricant layer or a protective layer on such recording layer to improve the running property or to enhance the running durability.
The protective layer for the metal thin film type-recording layer of a magnetic recording medium can be formed on the recording layer by coating an organic solvent solution of a thermoplastic resin, a thermosetting resin, a fatty acid, a metal salt of fatty acid, a fatty acid ester or an alkyl phosphoric acid ester over the recording layer, as described in Japanese Patent Provisional Publications No. 60(1985)-69824 and No. 60(1985)-85427.